Various exhaust gas purification systems for purifying nitrogen oxides in an exhaust gas discharged from an internal combustion engine, such as the engine of an automobile, have been developed. Patent Document 1 indicated below, for example, discloses a method for reducing nitrogen oxides, which comprises using hydrogen as a reducing gas, adding the reducing gas to an exhaust gas containing nitrogen oxides in a temperature region as low as 0 to 200° C. (low temperature region), and bringing the resulting mixture into contact with a catalyst to reduce and remove the nitrogen oxides in the exhaust gas.
Patent Document 1: JP-A-2-194818